The Canadian Prairies are highly dependent on precipitation especially during the growing season (May to August) when the majority of annual precipitation is received. Both the amount and temporal distribution of this growing season precipitation are critical to several environmental processes and economic activities and most notably, to agriculture. This study examines Canadian Prairie growing season precipitation variability and associated atmospheric circulation patterns for the period 1905-96. The majority of precipitation and atmospheric variables generally display substantial interannual as well as, interdecadal variability. The analysis identifies five decadal scale periods when the temporal distribution of precipitation (as measured by changes in the weighted centre of 30-day moving window precipitation totals) significantly shifted to earlier or later in the growing season. Monthly precipitation variability accounts for the observed shifts in temporal distribution including a significant increase in July/August rainfall during the mid to late 1940s, and a significant increase in May in the early 1970s. In addition, since 1980, July has replaced June as the month of maximum precipitation. Mid-tropospheric and sea level pressure atmospheric indices are developed to examine the large scale flow characteristics associated with precipitation variability. Indices at the 500 hPa and to a lesser extent, surface levels explain a substantial portion of variance in monthly precipitation, and are also generally related to shifts in the temporal distribution. Since 1974, May 500 hPa circulation has been characterized by a lower frequency of ridging (troughing) over western (eastern) Canada resulting in significantly higher May precipitation over the Prairies. Increases in July and August precipitation in the 1940s were associated with a major change in surface flow over the Prairies from predominantly westerly to easterly. Regression analysis reveals a difference in the large scale circulation patterns associated with precipitation during the early and late periods of the growing season. May and June precipitation is associated more with the merdionality of mid-tropospheric circulation, and the southerly component of surface flow. Conversely, July and August tend to be related to the westerly component at the surface, and differences in the strength of westerly flow between the northern and southern Prairies at the 500 hPa level